#include<bits/stdc++.h>
#define ONLINE_JUDGE
#define ll long long
#define ull unsigned long long
using namespace std;

const int N = 1e6 + 10, INF = 0x3f3f3f3f;

int n, m;
vector<int> g[N];
bool vis[N];

int bfs()
{
    queue<pair<int, int>> q;
    q.push({ 1,0 });
    while (!q.empty()) {
        auto [x, step] = q.front();q.pop();
        if (x == 1 && vis[x] == 1) return step;
        for (int y : g[x]) {
            if (vis[y]) continue;
            q.push({ y,step + 1 });
            vis[y] = 1;
        }
    }
    return -1;
}

void solve()
{
    cin >> n >> m;
    for (int i = 1;i <= m;i++) {
        int x, y;cin >> x >> y;
        g[x].push_back(y);
    }
    int ans = bfs();
    cout << ans;
}

signed main()
{
    // ios::sync_with_stdio(0);
    // cin.tie(0);cout.tie(0);
#ifndef ONLINE_JUDGE
    std::istringstream in(R"()");
    std::cin.rdbuf(in.rdbuf());
#endif
    int T = 1;
    //cin>>T;
    for (int i = 1;i <= T;i++) {
        solve();
    }
}